Technical Field
This disclosure relates to methods and systems for drilling boreholes in the earth in general, and more specifically, to methods and systems for drilling blast holes of the type commonly used in mining and quarrying operations.
Background
Various systems and methods for drilling boreholes are known in the art and have been used for decades in a wide variety of applications, for example, from oil and gas production, to mining, to quarrying operations. In mining and quarrying operations, such boreholes are typically filled with an explosive that, when detonated, ruptures or fragments the surrounding rock. Thereafter, the fragmented material can be removed and processed in a manner consistent with the particular operation. When used for this purpose, then, such boreholes are commonly referred to as “blast holes,” although the terms may be used interchangeably.
A number of factors influence the effectiveness of the blast, including the nature of the geologic structure (i.e., rock), the size and spacing of the blast holes, the burden (i.e., distance to the free face of the geologic structure), the type, amount, and placement of the explosive, as well as the order in which the blast holes are detonated. Generally speaking, the size, spacing, and depth of the blast holes represent the primary means of controlling the degree of rupture or fragmentation of the geologic structure, and considerable effort goes into developing a blast hole specification that will produce the desired result. Because the actual results of the blasting operation are highly correlated with the degree to which the actual blast holes conform to the desired blast hole specification, it is important to ensure that the actual blast holes conform as closely as possible to the desired specification.
Unfortunately, however, it has proven difficult to form or drill blast holes that truly conform to the desired specification. First, a typical blasting operation involves the formation several tens, if not hundreds, of blast holes, each of which must be drilled in proper location (i.e., to form the desired blast hole pattern) and to the proper depth. Thus, even where it is possible to achieve a relatively high hole compliance rate (i.e., the percentage of blast holes that comply with the desired specification), the large number of blast holes involved in a typical operation means that a significant number of blast holes nevertheless may fail to comply with the specification. In addition, even where blast holes are drilled that do comply with the desired specification, a number of post-drilling events, primarily cave-ins, can make a blast hole non-compliant. Indeed, such post-drilling events can be major contributors to blast hole non-compliance.
Still further, because of the large number of blast holes that are typically required for a single blasting operation, methods are constantly being sought that will allow the blast holes to be formed or drilled as rapidly as possible. As with most endeavors, however, there is an inverse relationship between speed and quality, and systems that work to increase speed at which a series of blast holes can be drilled usually come at the expense of hole quality. Consequently, there is a need for methods and systems for forming blast holes that will ensure consistent blast hole quality while minimizing the adverse effects on the speed of blast hole formation.
There is a desired ratio of penetration rate per drill bit revolution where the drill bit carbides penetrate and fracture the rock efficiently, resulting in desirable drilling speed and bit-wear characteristics. This ratio is referred to as the depth of cut (DOC). An optimum rate of penetration (ROP) for drilling efficiency can be calculated by multiplying the maximum rotation speed by the DOC. Prior art methods have used a simple feedback loop to adjust the feed force applied to the bit to maintain an assumed optimum penetration rate. (Feed force applied to the bit being generally proportional to the achieved rate of penetration.) In this application the terms “feed force” and “weight-on-bit” or “WOB” are used interchangeably.
However, at times it may be desirable to sacrifice the efficiency of the ideal depth of cut to achieve a higher penetration rate. Conversely it may be desirable to sacrifice rate of penetration to achieve longer consumable life; that is, the life of the drill bit. Also, such prior-art methods give an optimum DOC at a single penetration rate. What is needed is a method of monitoring and adjusting these opposing goals to achieve optimum drilling efficiency over a wide range of penetration rates, depending on local drilling conditions. As used in this application, the term “drilling efficiency” is not a precisely-defined term, but refers to the optimum ratio of the rate of penetration of the bit to the energy expended for extraction of a given volume of rock, taking into consideration also the amount of bit wear in such extraction.
Although this application is focused on solving problems in blast hole drilling operations, the disclosure and claims are equally applicable to the drilling of boreholes in other fields, such as oil and gas drilling.